Antennas that require large focal plane offsets, necessary for beam scanning, also require long focal lengths (high F-numbers) to minimize aberrations for acceptable beam quality of the antenna pattern. High F-numbers increase the antenna length or thickness to dimensions that in many cases are unacceptable.
There are several forms of "polarization twist" Cassegrain antennas presently in use today. In one type, the secondary sub-reflector reflects the focused rays from the primary paraboloidal reflector back to a focal point in the region of the primary reflector's vertex. By twisting the polarization, the blockage as a result of the secondary reflector shadowing the rays on the primary parabola is greatly reduced. This known approach incorporates small F-number primary apertures (0.3 to 0.4) to minimize the antenna thickness. This is followed by a 2 to 4 times gain small diameter hyperboloidal secondary reflector to increase the ray path length to the focal point near the primary vertex. The quality of the antenna pattern, which includes gain and sidelobes over the scanned angle, is strongly influenced by the primary reflector's F-number, not the effective F-number. Assuming a known Cassegrain antenna with a 0.35 F-number, the field-of-view would be limited to 4 beamwidths (to achieve same beam quality as the 9 beamwidth embodiment described below in accordance with the invention) and have about a 10% greater length.